Each character is a typical prior art display usually comprising seven separate LED segments. Each segment is powered to provide substantially the same light emission even though fewer segments are illuminated in some characters than in others.
In a display of a series of different characters, characters having more segments illuminated than others appear to be brighter to most observers. This apparent difference in brightness is emphasized when a character having a few segments illuminated is immediately adjacent a character having many segments illuminated. Thus an 8 generally appears to be brighter than a 1.
The present invention compensates for this apparent brightness difference to provide a display of characters having apparently uniform brightness regardless of the number of segments illuminated per character or the number of different characters in the display. This compensation is achieved by including a power control circuit in the anode drive circuitry for each LED. The brightness per character is reduced approximately as the square root of the number of segments illuminated. Not only is the power required to drive characters having several segments illuminated substantially reduced using this scheme, but the necessary power drive capability of the driver circuitry is similarly reduced.